Ceramic materials have desirable properties for many applications. Zirconia ceramics are especially desirable, because they are generally harder, stronger, more impact resistant, and less reactive with materials in the environment, such as body fluids, than many other ceramics, making them suitable materials for applications where the ceramic components are subjected to harsh environments.
Most untreated oxide ceramic materials, such as zirconia, are light in color. Processing of such ceramic materials, including assembly of oxide ceramic components into assemblies including different materials, may produce discoloration of the ceramic material. Sealing of ceramic components to metallic components, for example, often produces discoloration of the ceramic material. Discolored ceramic materials may exhibit the same desirable properties as ceramic materials having a uniform coloration, but they are far less desirable from a commercial standpoint. It is therefore desirable to darken ceramic materials so that discoloration is not apparent. Darkened ceramic materials may also be preferred for certain applications.
U.S. Pat. No. 5,711,906 discloses a method of manufacturing a black zirconia-based article that incorporates a stabilizer and a pigment. Zirconia powder is mixed with a stabilizer, a pigment formed by a cobalt and iron oxide powder or by a spinel CoFe.sub.2 O.sub.4 powder, and a binder, the mixture is granulated, and the desired article is molded, sintered and polished. The '906 patent also discloses that molded, stabilized zirconia elements will become black if sintered at a temperature of 1,400.degree. to 1,600.degree. C. in a non-oxidizing atmosphere.
U.S. Pat. No. 4,656,145 discloses a black zirconia decorative article produced by sintering an unsintered molded body comprising a zirconia matrix and a stabilizer in a non-oxidizing atmosphere, such as a carbon-containing atmosphere to produce a blackened, sintered article. Blackening agents include zirconium carbide, carbon or a reduction product of zirconia in an amount of 0.001 to 1% by weight. Sintering promoters such as silica, alumina or titania may be incorporated in the unsintered molded body.